1. Field of the Invention
This invention relates in general to the field of maneuver control of a vehicle traveling through a fluid environment and, more particularly, to a maneuver strategy to improve rapid turning capability.
2. Description of the Related Art
FIG. 1 illustrates a conventional vehicle 50 capable of traveling through a fluid medium, e.g., air, water, and plasma. The particular embodiment of the vehicle 50 illustrated is a missile system designed to travel through air. Although the invention is disclosed herein primarily in the context of a missile system, it is to be understood that the invention is not so limited. The invention is disclosed in the context of a missile system for the sake of clarity and to facilitate understanding of the invention. Those skilled in the art having the benefit of this disclosure will recognize that the teachings herein can be extrapolated to other vehicles capable of traveling through other fluid media, e.g., a torpedo traveling through a body of water.
The missile system 50 illustrated in FIG. 1 can be described as an elongated body 100 that travels through a fluid medium, typically the earth's atmosphere. The missile 100 has a forward section 102 and an aft section 104 divided by the missile body 100's center-of-gravity 105, the forward section 102 having a control device 110. Missile control devices can fundamentally be categorized as forward aerodynamic, e.g., canards; forward propulsive, e.g., a thruster; aft aerodynamic, e.g., fins; and aft propulsive, e.g., thrust vector control. Thus, it will be apparent to those skilled in the art having the benefit of this disclosure that other control devices alternative to those shown in FIG. 1 are possible and may even be desirable depending on the particular application for the missile system 50. For instance, the forward control device 110, a set of thrusters in the embodiment illustrated, may be implemented with canards, instead. Control devices may also be used in tandem.
Certain aspects of a missile's maneuverability are typically expressed in terms of angles. FIG. 2 illustrates the angle nomenclature typically used by those in the art to describe missile maneuverability. The nomenclature is frequently defined in terms of an inertial coordinate system that is fixed relative to the earth. An axis in this coordinate system is an "inertial axis." For simplicity, a single pitch plane will be used to define the nomenclature although those in the art will recognize that more than one plane might be relevant in some circumstances. The angle 200 between the nose 215 of the missile system 50 and the inertial axis 205 is called the pitch angle. The angle 210 between the nose 215 of the missile 50 and the missile's velocity vector 220 is called the angle-of-attack. The angle 225 between the velocity vector 220, i.e., the magnitude and direction of the missile's travel, and the inertial axis 205 is called the flight path angle. Thus, as shown in FIG. 2, for pitch plane maneuvers, the pitch angle 200 is the sum of the angle-of-attack 210 and the flight path angle 225.
Missiles are typically directed toward a target. This target may be in the form of a set of inertial coordinates or a physical object or a combination of both. If the missile is stationary at launch, its initial flight path angle 225, is aligned with the missile's nose or equal to its pitch angle 200. If the missile is air launched, the missile's initial flight path angle is inherited by the flight path angle of the host aircraft and launcher imposed dynamics. The initial flight path angle 225 is typically not equal to the flight path angle desired to reach the target. The missile must maneuver to change its current flight path angle 225 to its desired flight path angle.
Typically, a missile system relies on aerodynamic force to maneuver. In the atmosphere, the amount of force the missile system can receive from aerodynamics exceeds the amount of lateral control force that can be packaged in the missile at altitudes of less than about 20 km. Because of this, the primary purpose of control mechanisms is generating a moment to rotate the nose of the missile to produce the angle-of-attack 210. The angle-of-attack 210, in turn, generates an acceleration normal to the missile body which results in a rotation of the missile's flight path angle 225. Maintaining an adequate missile velocity to supply the necessary aerodynamic forces and moments is required to maneuver in the conventional manner.
More particularly, and referring again to FIG. 1, a force generated by a control device, such as the forward control device 110, generates a moment, or torque, on the missile body 100 about the center-of-gravity 105 of the missile body 100. Below an altitude of approximately 20 km, a missile system's primary source of transverse acceleration is the aerodynamic force resulting from the missile body 100's angle-of-attack. Flight control devices, e.g., forward thrusters 110, are commanded by control processes encoded in and executed by computational devices carried by the missile system to generate moments and forces and obtain this angle-of-attack.
FIG. 3 is a functional block diagram of a missile control system 301 for the conventional missile system 50 of FIG. 1. Block 330 represents the physical system, i.e., the missile, and incorporates descriptions of all vehicle subsystems including, for example, control actuation, propulsion, and inertial measurement systems as well as the aerodynamic configuration. The missile guidance logic 310 determines a desired kinematic trajectory by commanding a dynamic response that results in a change to the missile's flight path. The vehicle's measured dynamic response is shown as feedback signal 305. This signal encodes, for example, a measurement of the missile's rotational rates, translational rates, accelerations, and inertial orientation. In general terms, the autopilot controller 325 is a control process that uses the difference between desired missile responses, e.g., rates, accelerations, or attitudes, and the measured responses to define a set of error signals 320 which the controller uses to encode control commands 330 for a control actuation device, e.g., thruster level, that is a part of the vehicle dynamics and kinematics 300.
The energy necessary to operate a flight control device is very precious in an operational sense. A missile system can carry only a limited amount of energy, e.g., energy stored as rocket fuel, batteries, etc. Strategically, altering the amount of energy that may be provided can affect the missile system's range and performance against a target. Tactically, the energy is conserved as much as possible so that the missile system can retain its ability to maneuver from the time it is deployed to the time it reaches the target. Thus, the energy necessary to operate the flight control device is an important design consideration for both the physical structure of the missile system and its operation.
Air launched missiles are typically mounted in a forward facing direction where the nose of the missile is aligned with the nose of the host aircraft. A missile able to reverse the direction of its flight path, e.g., 180.degree. turn, is able to provide a capability to destroy rearward approaching aircraft before they are able to be within firing range of the host aircraft. A combat advantage is also provided in a fly-by scenario where an enemy aircraft passes the host aircraft traveling the opposite direction. If the missile can turn rapidly enough, the enemy aircraft can not escape being defeated. Thus, the time which the maneuver consumes, the velocity of the missile after the maneuver, and the turning radius or distance which the missile deviated from its desired flight path is critical to the life and death nature of the engagement.
Thus, one application exhibiting an immediate need for improved efficiency in maneuvering is an air launched interceptor missile. The intuitive approach to reversing an air launched interceptor missile's flight path based on conventional maneuver control is a steady high-g turn. The missile system applies control force to rotate the missile system's nose to a desired angle-of-attack to achieve an aerodynamic normal force providing a lateral component of acceleration to rotate the missile system's velocity vector and therefore its flight path angle. During the intuitive maneuver, forward velocity is necessary to produce aerodynamic force and to maintain an angle-of-attack. The angle-of-attack is typically limited to reduce the parasitic effects of aerodynamic axial forces, to maintain stability of the vehicle during the sustained maneuver, and to constrain aerodynamic loading beneath the structural limit of the missile body.
FIG. 4 illustrates the first 1.25 seconds of an exemplary trajectory of the missile system 50 in FIG. 1 performing the sustained maneuver in the vertical plane. The inertial orientation of the missile system 50 is shown at intervals of 0.25 seconds during the maneuver. As is evident from FIG. 4, this maneuver requires a large turning diameter. FIG. 5 graphs the total missile velocity as a function of the inertial x-axis or downgrade progress for the first five seconds of flight in the maneuver of FIG. 4, illustrating the energy expended in the direction opposite the desired maneuver direction.
One missile system developed specifically for such demanding maneuvers is the Python 4 manufactured by Rafael Industries and sold by the Israeli government, a line drawing of which is shown in FIG. 6. The Python 4 includes twenty-one (21) flight surfaces to achieve this performance-an inordinately high number of flight surfaces for a missile system that makes it expensive to manufacture and difficult to maintain. One reason for the Python 4's design complexity is the demands of implementing the intuitive approach for reorienting the missile described above and illustrated in FIG. 4. The Russian AA-11 missile system shown in FIG. 7, likewise, implements this intuitive approach to reorient the missile system to perform a 180.degree. turn.
The present invention is directed to improving performance and to overcoming, or at least reducing the effects of, one or more of the problems set forth above.